For engaging hook and loop components of a fastener, an array of fastener hook elements formed on a common supporting member is pressed face-wise against a field of loops. The quality of the fastening is judged principally by its peel and shear strengths, strengths that are affected by many aspects of the design. Of particular concern are the number of hook elements present, the probability that each given hook element will be engaged in a loop, and the amount of resistance provided by that hook element before it deflects sufficiently to release the loop. These aspects are interrelated.
The probability of engagement of a given fastener hook element with a loop generally increases with the depth of penetration of the hook elements into the loop material. This depth of penetration is dependent upon the maximum penetration depth permitted by the overall hook component, ordinarily set by the height of its hook elements, and upon the frontal resistance to penetration. This frontal resistance is determined by the size of the frontal profile (“foot print”) of the individual hook elements confronting the loop material and by the areal density of the hook elements on the hook component.
The degree of resistance offered by a hook element to disengagement from a loop concerns the force required to bend the hook element sufficiently that the loop slips from the hook head. For hook elements that are otherwise the same, the taller the stem of the hook element in the plane of loading, the more easily it is bent to release the loop, hence the weaker its engagement with a loop. Similarly, the shorter the stem, the more resistance it offers to such loop release.
A hook element formed by a stem and an overhanging hook head typically has a directional quality that affects its resistance to disengagement. When the loop is pulled in the direction against the stem of the hook element, the hook element resists disengagement more effectively than when it is pulled in the opposite direction, away from the stem. Where resistance to unfastening against motions in both directions is desired, each fastener element may be provided with head portions that protrude in opposite directions. Alternatively, single direction hook elements that are closely adjacent may be oriented in opposite directions, decreasing the frontal area of each hook element, but requiring more fastener elements for similar strength properties. Hook components of these kinds may be designed especially for strength in the plane that includes the height of the fastener elements and their heads. It is usually desirable that the fastener elements also have significant strength in the orthogonal plane that includes the height of the elements, because some fraction of loading may occur in that plane.
There are other hook-loop performance criteria recognized in the field. As well, the conditions of use affect performance, for example, shifts can occur between joined components during normal use of a product, altering the relationship between loops and the hook elements, to bring more or fewer loops under hook heads.
Furthermore, a hook design, to be practicable, must also satisfy concerns about manufacturability, durability of tooling and cost of manufacture.
In the face of demands for less expense and better performance, a significant challenge confronts any attempt to improve hook fastener components. Such demands occur in particular for mass-produced, disposable products such as infant care products, personal care and medical products and packaging. In such products, the loop component is formed of inexpensive nonwoven materials which are typically difficult to engage satisfactorily with fastener hooks. Nonwoven loop materials are constructed, for instance, of a layer of fibers or filaments that have relatively raised or lofty loop regions between regions secured by adhesives or self-adhesive bonds. There is particular need in these cases for the hook component to be low-cost while having dependable fastening properties.